((hugs))
At 4 months, you would probably want to start making the transition to 4hr EASY. At this age, we were doing 2hrs A time and feeding every 3.5-4hrs. If your LO is breastfed, you may not be able to get him to stretch 4 hrs but at least aim at spreading the E's out to 3.5hrs.
Time to Transition - 3hr, 3.5hr or 4hr EASYAt 4 months, we were also struggling with some short naps (mainly the afternoon nap) but we managed to get 1 long nap in the morning. If you stretch his first A to 2hrs, you may find that he will want to sleep longer and even if you get 1 good nap a day, that's fine too. He's probably outgrown the 4th nap.
You don't need to move onto formula but definitely try and space out the feeds so he takes more in. We only managed to get 2 solid naps when we transitioned to 2 naps (around 5 months)... short naps could also be developmental but see how you go trying to push the first A and get him to take a longer nap in the morning.
When he wakes, I would also just leave him in the cot. I used to leave J in the cot for up to an hour from when nap started (so extra 15mins if the nap was 45mins) and she would sometimes put herself back to sleep for another 45mins. Obviously if she was crying and it wasn't a mantra cry, I would go to her and try to shh/pat her back to sleep but again just for another 15mins and if S didn't happen, I would move on with our day and give her some more A before giving her E.
There is also a massive growth spurt and sleep regression at around 4 months so hang in there, it will get better